The present invention relates to a method of manufacturing an optical memory element enabling at least one of recording, reproduction and erasure of information and a photomask used directly for manufacture of the optical memory element.
In recent years, optical disks for performing reproduction, etc. of information by using laser beams have been greatly developed. In an optical disk, a thin film of recording medium is deposited on a plastic substrate made of polycarbonate or acrylic resin and a protective member is formed on the thin film in close contact therewith. A spot of laser beams converged to a diameter of about 1 .mu.m is irradiated onto the optical disk rotating on an optical disk player such that recording, reproduction and erasure of information are performed.
Meanwhile, when the optical disk is being rotated by a rotary motor, tracks on the optical disk are likely to vibrate in axial and radial directions of the rotary motor due to distortion of the optical disk or run-out of the rotary motor. Therefore, unless a countermeasure for preventing this vibration of the optical disk is taken, the spot of laser beams cannot follow the tracks accurately. This is partly because the tracks on the optical disk are formed at an excessively minute pitch of about 1.6 .mu.m.
Thus, conventionally, an optical disk device is usually provided with a system for causing, through detection of vibration of the optical disk, the spot of laser beams to automatically follow signal pits recorded along the tracks, i.e. a so-called servo system. The servo system is of either a continuous servo type or a sampled servo type.
In the continuous servo type, grooves are formed spirally or coaxially on the optical disk such that the spot of laser beams follows the grooves. On the other hand, in the sampled servo type, pits are formed discontinuously on the surface of the optical disk such that the pits cause the spot of laser beams to follow the tracks.
FIG. 1 shows a known optical disk of sampled servo type. In the known optical disk, staggered pits (wobble pits) 2 and 3 are alternately provided at opposite sides of a track centerline 1, while a clock pit 4 is provided on the track centerline 1. As shown in FIG. 2, if the spot of laser beams travels along the track centerline 1 rightwards in FIG. 1, signal strength obtained at the time when the spot has passed through the left pit 2 becomes identical with signal strength obtained at the time when the spot has passed through the right pit 3. On the contrary, if the spot of laser beams travels along a line deviating from the track centerline 1, the above two signal strengths become different from each other. Hence, as shown in FIG. 3, if a difference between the two signal strengths is detected by a signal processing circuit 12, a signal (radial error signal) indicating the direction and how far the spot of laser beams deviates from the track centerline 1 can be obtained such that servo control in the radial direction is performed in accordance with this signal. Meanwhile, timing of sampling is determined by the clock pit 4. Servo control in the focusing direction is performed by sampling a signal obtained at the time when the spot of laser beams is located at a portion A in FIG. 1. Also in this case, timing of sampling is determined by the clock pit 4.
Meanwhile, such optical disk (optical memory element) is manufactured by two methods. In one manufacturing method, plastic such as polycarbonate, acrylic resin, etc. is subjected to injection molding. The other manufacturing method is employed for manufacture of the continuous servo type optical disk. In the latter manufacturing method, pits or grooves are directly formed on a glass substrate.
The latter manufacturing method of the optical disk is performed as shown, for example, in FIGS. 4a to 4f. Initially, as shown in FIGS. 4a and 4b, a photoresist 6 is coated on a glass substrate 5. Then, as shown in FIG. 4c, a photomask 7 having a desired pattern formed thereon is laid on the photoresist 6 in close contact therewith. Subsequently, exposure of the photoresist 6 is performed from above the photomask 7 so as to print the above mentioned pattern onto the photoresist 6. Then, as shown in FIG. 4d, the pattern printed on the photoresist 6 is developed so as to be transferred onto the photoresist 6 such that a photoresist pattern 6' is obtained. Subsequently, after the glass substrate 5 has been etched as shown in FIG. 4e, the photoresist pattern 6' is removed as shown in FIG. 4f and thus, concave and convex portions are formed on the surface of the glass substrate 5 into the desired pattern. Therefore, for example, thus obtained grooves 11 are formed concavely relative to the remaining portions on the surface of the glass substrate 5.
In the above described manufacturing method, portions having a high light transmittance and portions having a low light transmittance are formed on the photomask 7. The portions having a high light transmittance correspond to portions of the glass substrate 5, at which the pits or the grooves are formed, while the portions having a low light transmittance correspond to portions of the glass substrate 5, at which the pits or the grooves are not formed. One reason for such correspondence is that a resist having a high resolution on the order of submicrons required of the optical disk is restricted to a positive type. Namely, after development, the resist should not remain at the portions of the glass substrate 5, at which the pits are formed. Thus, if the pits are formed by using such positive type resist, portions of the photomask 7 corresponding to the pits of the glass substrate 5 should be the portions having a high light transmittance. Another reason for the above correspondence is linked with a manufacturing method of the photomask 7. In order to manufacture the photomask 7, a metallic film 9 is initially formed on a transparent substrate 8 as shown in FIG. 5a. Then, as shown in FIG. 5b, a photoresist 10 is coated on the metallic film 9. Subsequently, as shown in FIG. 5c, laser beams are irradiated onto the photoresist 10 so as to record a desired pattern on the photoresist 10. Then, after a photoresist pattern 10' has been formed by development as shown in FIG. 5d, the metallic film 9 is etched so as to obtain a pattern 9' of the metallic film 9 as shown in FIG. 5e. Subsequently, as shown in FIG. 5f, the photoresist pattern 10' is removed and thus, the above described photomask 7 is obtained.
In this manufacturing method of the photomask 7, it is needless to say that the photoresist 10 is required to have a high resolution and therefore, should be of positive type. Therefore, the portions of the photomask 7 corresponding to the portions of the glass substrate, at which the pits or the grooves 11 are formed, are necessarily the portions having a high light transmittance.
By forming on the thus obtained glass substrate 5 a recording medium enabling recording, reproduction and erasure of information, for example, a recording medium made of material utilizing phase transition between crystalline phase and amorphous phase or magnetooptical material based on perpendicular magnetization, an optical memory element is obtained. Since the above described glass substrate 5 is employed in the optical memory element and glass itself has low hygroscopicity and low gaseous transmission, deterioration of the recording medium with time is restricted, so that a highly reliable optical memory element can be obtained.
Meanwhile, when the photomask 7 is laid on the photoresist 6 coated on the glass substrate 5 so as to be brought into close contact with the photoresist 6, it is necessary to judge whether or not the photomask 7 is properly held in close contact with the photoresist 6 by visual inspection from above the photomask 7. If dust is present between the photomask 7 and the photoresist 6, interference fringes may be observed by interference of light when the photomask 7 is laid on the photoresist 6 in close contact therewith. If the glass substrate 5 having the interference fringes is subjected to exposure without eliminating the interference fringes, defective pits or grooves are formed at portions of the glass substrate 5 corresponding to the interference fringes and thus, various servo systems or reproduction signals of the optical disk are adversely affected.
Namely, it is essential for manufacture of the optical disk that inspection of close contact of the photomask 7 with the photoresist 6 is performed securely. In manufacture of the continuous servo type optical disk, since the portions of the photomask 7, which have a high light transmittance, are relatively large in number, the above inspection can be performed relatively easily.
On the other hand, in the case of the sampled servo type optical disk, since the portions of the photomask 7, which have a high light transmittance, are restricted to portions corresponding to the staggered pits 2 and 3 and the clock pits 4, light transmission of the photomask 7 as a whole is low and thus, it is difficult to perform the above inspection. Therefore, such an inconvenience is incurred that even if dust or the like is present, dust or the like cannot be found. Furthermore, such a problem arises that since inspection of close contact of the photomask 7 with the photoresist 6 is insufficient, defective optical disks may be manufactured.